Diesel engines provide lower emissions and increased fuel economy as compared to gasoline engines; however, environmental and health hazards are posed by diesel exhaust emissions. In particular, diesel particulate filters control particulate emissions by physically trapping soot particles in their structure.
Diesel particulate filters are preferably constructed as honeycomb wall-flow monoliths, which allow exhaust gases to flow through their porous ceramic walls, while any particulate present in the exhaust gas is collected on the upstream side of the wall. Once a predetermined condition is met, the filter may be cleaned by a regeneration cycle, during which the temperature of the exhaust gas is high enough to ignite and burn out any soot particulate. This regeneration cycle reduces the backpressure of the diesel particulate filter near to that of a new filter.
The surface of the walls or the porous interior of the walls of diesel particulate filters may include a catalyst washcoat containing catalysts such as platinum (Pt), palladium (Pd), iron (Fe), strontium (Sr), cerium (Ce), or other transition metal or rare earth elements. Such catalysts can operate to lower the temperatures required for the regeneration of the filter, and to convert hydrocarbons, carbon monoxide and/or oxides of nitrogen in the exhaust gases into water vapor, carbon dioxide and/or harmless nitrogen compounds.
One preferred material for the manufacture of high temperature components is cordierite (Mg2Al4Si5O18), a magnesium aluminum silicate that often includes low levels of iron or other impurities. Cordierite ceramics have a low coefficient of thermal expansion (CTE), high strength and are resistant to thermal shock. Cordierite materials are commercially manufactured by mixing a raw batch including talc, alumina, aluminum hydroxide, kaolin clays, and silica. The batch is then blended with a binder such as methylcellulose and lubricants such as sodium stearate to form a plastic mixture that is formed into a green body, dried, and reaction-sintered.
Typically, high porosity honeycomb bodies for applications such as particulate filters include large concentrations of pore formers such as graphite, starch, or other pore forming materials. The use of graphite is favored, but presents many difficulties in the manufacture of the body. Typically, bodies with high loadings of graphite have a decreased dielectric constant, which decreases the efficiency of dielectric or microwave drying. Another difficulty with graphite is the exothermic reaction caused during initial heating in the range 650° C. to 1000° C., which initiates combustion of the graphite. Careful control of the firing process during the graphite burnout stage is required in order to control the graphite combustion rate.
Commonly assigned U.S. Pat. No. 6,864,198 discloses a method of forming cordierite honeycomb structures, including honeycombs meeting specific porosity and pore size distribution requirements for low-pressure drop filters, made in accordance with the patent through the use of batches containing fine talc. However, one difficulty with highly porous cordierite ceramics such as these can be reduced strength, since the high porosities required to achieve low pressure drops in exhaust filters reduce the structural density and/or toughness of the honeycomb wall structures. Other methods for manufacturing highly porous cordierite honeycombs are disclosed in U.S. Pat. Nos. 5,258,150 and 6,818,580, the latter patent disclosing cordierite-forming batches wherein alumina of moderate particle size comprises one of the batch constituents.
In light of the above concerns, cordierite honeycomb articles having increased or substantially similar porosity that can be made with lower concentrations of pore former, especially including lower graphite loadings, are much sought after. Such decreases in pore former loadings cannot, however, come at the expense of other important properties directly impacted by porosity, such as, for example, clean (soot-particle-free) pressure drop and filtration efficiency. Accordingly, there remains a need for cordierite ceramic honeycombs that exhibit high porosity and filtration efficiency as well as high strength, and that can be made while reducing the amount of pore former used in the cordierite batch material.